A package as one example includes a tray for occupancy by an object to be heated, such as food. The tray has a top face which is sealed to a sheet to hermetically close an opening of the tray. A user can heat such a package using a heating means, such as a microwave oven, to cook food, for example, that is an object to be heated.
When the object to be heated generates steam with the heating of the package, the inner pressure of the tray increases to push up the sheet due to inflation. Then, when the inner pressure has increased to an extent that the sheet can no longer endure the inner pressure, without the tray's being deflated, the sheet may be broken due to the excessive inflation. For this reason, a package whose inner pressure increases due to heating is provided with a steam passage portion to externally release the steam from inside the tray. The steam passage portion is formed in a part of a seal portion where the sheet is sealed to the tray.
PTL 1 discloses an example of such a package. The package includes a seal portion which is partially provided with a steam passage portion (a protrusion) protruded from an outer edge of the tray toward an opening. Since the steam passage portion is in a shape with a pointed tip, the inner pressure of the tray is easily concentrated on the steam passage portion, compared to a linearly-shaped portion of the seal portion. Therefore, when the inner pressure of the tray has increased with the heating of the package, the steam passage portion will peel off earlier than other parts of the seal portion. As a result, a steam passage is formed in the peel-off portion so that the interior and the exterior of the tray can communicate with each other. Thus, the steam inside the tray is externally released through the steam passage, and unlikely to create a state where the sheet is excessively pushed up due to inflation.